In accordance with an embodiment of the invention, a push-pull drive circuit includes a first transistor circuit stacked in series with a second transistor circuit. One or the other of the first and second transistor circuits is atop the other. The transistor circuits alternately push and pull current through an output between the first and second transistor circuits. Each transistor circuit may be a single transistor, a Darlington pair of transistors or other circuit arrangement for pushing or pulling current. Bias currents for each of the first and second transistor circuits is provided. Operation of each transistor circuit is controlled by a sink transistor which selectively diverts the bias current away from its respective transistor circuit. When the sink transistor for the first transistor circuit diverts bias current from the first transistor circuit, it at the same time diverts the bias current from the base of the sink transistor for the second transistor circuit. It may in certain applications also be useful to provide a capacitor between the sink transistor for the first transistor circuit and the sink transistor for the second transistor circuit to more rapidly provide current to the second sink transistor when the first sink transistor switches from diverting current to allowing current to the first transistor circuit. In addition, it may be desirable to provide a capacitor having one end coupled to the bias current for the first transistor circuit and its other end coupled to ground so as to slow switching on of the first transistor circuit when the first sink transistor switches from diverting current to allowing current to the first transistor circuit.
In accordance with a method of changing states of first and second transistor circuits that form a push-pull drive circuit, a first bias current is provided to the first transistor circuit. A bias current is diverted from the second transistor into a sink transistor circuit to keep that second transistor circuit from conducting output current. The method then proceeds with simultaneously diverting the first bias current from the first transistor circuit and from the sink transistor. In this manner, the first transistor circuit is kept from conducting output current and then the sink transistor will allow the second bias current to drive the second transistor circuit.
According to a method of a further embodiment of the invention, a low input signal to a first sink transistor is clamped in relation to a voltage at an emitter of the first sink transistor. A first bias current is provided to the base of the first transistor circuit. A second bias current is diverted from the base of the second transistor circuit into a second sink transistor. The method goes on to raising the input signal to the first sink transistor to a high input signal. The first sink transistor responds by simultaneously diverting the first bias current from the base of the first transistor circuit and from the base of the second sink transistor. This switches off the first transistor circuit and allows second bias current to drive the second transistor circuit on.
A further embodiment of the invention is disclosed in which each of the first and second transistor circuits has an associated bias current. Also, each circuit has an associated sink transistor for selectively diverting bias current from its associated transistor circuit. The first sink transistor is arranged to divert bias current from its associated first transistor circuit and from the base of the other sink transistor. A ground referenced transistor has a collector coupled to the base of the first sink transistor. A diode clamp is connected between floating ground and the collector of the ground referenced transistor. Each of the sink transistors is also connected to the floating ground through their emitters.
A switching signal input circuit is also described for use with the drive circuit. An input transistor is responsive to a high or low voltage responding by sinking or allowing a first bias current to flow into a first transistor. The first transistor and a second transistor share a common base and both have emitters coupled to ground. A second bias current coupled to a control input of a drive circuit is controlled by the second transistor through its collector. The second transistor sinks the second bias current when the first bias current is allowed to flow into the first transistor of the switching signal input circuit and allows the second bias current to drive the control input of the drive circuit when the input transistor is sinking the first bias current. In addition, a diode clamp may be connected between the collector of the second transistor and the floating ground of the drive circuit.
Embodiments of the invention advantageously prevent cross-conduction between the stacked transistor circuits in one or both directions. Propagation delay of the switching signal is advantageously reduced by clamping a low control input to a floating ground from the drive circuit. Other objects and advantages of the invention will become apparent during the following description of the presently preferred embodiment of the invention taken in conjunction with the drawing.
The present invention relates to a push-pull drive circuit. In particular, attention is given in the circuit to prevention of cross-conduction and reduction of propagation delay for the switching signal input.
Drive circuits arranged with series stacked transistors to drive a power transistor are well known in the art. These typically operate as a push-pull drive circuit in which the top transistor provides current out to the power transistor in a first state of the circuit, and in the second state of the circuit the bottom transistor pulls current from the power transistor to a ground. When current is flowing through one of the stacked transistors in saturation, power dissipation is low because the saturation voltage is very low. When a transistor is off, no current is flowing and no power is used by that transistor. However, during the transitions between the pushing and pulling states, power dissipation will increase particularly if both transistors in a stack are conducting at the same time. In order to minimize this power dissipation, it is highly desirable to prevent the transistors in the stack from being conductive at the same time. An example of a cross conduction prevention circuit is shown in U.S. Pat. No. 5,896,058. That patent discloses a complex circuit embodiment in which pairs of transistors arranged as NOR gates and a comparator are all provided to prevent cross conduction between stacked transistor circuits.
Such drive circuits may be used in a variety of applications. For example, a push-pull drive circuit can be used to drive a power transistor or a FET within a buck regulator. In order to maintain tight regulation, it is desirable that the switching signal to the drive circuit be acted upon without undue delay.